1. Technical Field of the Invention
The present invention relates to integrated circuits, and in particular integrated memory devices of the “One-Time Programmable” type, also known as “OTP” memories.
2. Description of Related Art
The OTP memory is well known to the person skilled in the art. It comprises fusible elements, such as capacitors, that are “blown” electrically, that is to say the dielectric of the capacitor is electrically impaired in an irreversible manner, so that the latter then behaves as a resistance of small value.
The reliability of memory cells in programming mode is a significant element. This reliability is affected, as regards the cells not selected during programming, by the stresses on the gate oxide of the access transistor of the memory cell. These stresses may be tolerated on MOS access transistors of the “field gradient” type (also known as “drift transistors”).
These stresses may also be limited using access devices forming cascode arrangements or else using high-voltage switches disposed in parallel with the word lines of the memory.
As regards the cells selected during programming, their reliability is affected by the phenomenon of hot carrier degradation.
This reliability problem arises in particular for CMOS technologies at 65 nanometers and less, by virtue in particular of the strong current passing through the access transistor exhibiting a strong drain/source voltage. A solution for reducing this strong current consists in using access transistors having significant channel lengths, but this has a negative impact on the surface area of the memory cell.
Another solution consists in reducing the programming voltage but this has a negative impact on the programming duration.
Moreover, it is currently difficult to detect the actual instant of programming, that is to say the moment at which the dielectric zone has actually been “blown”. So, a sufficiently long programming duration is currently fixed, on completion of which it is considered that the programming of the cell has actually taken place in a quasi certain manner. However, it turns out that this duration is not optimized since in most cases, the impairment of the dielectric has taken place well before the expiry of this duration.